1. Field of the Invention
The invention relates to an abnormality determination system for a multi-cylinder internal combustion engine having a vaporized fuel purge device that introduces the vaporized fuel that has occurred in a fuel tank into an intake passage of the engine, the abnormality determination system being capable of determining whether an intake air amount abnormality that “the degree of imbalance in the amount of intake air to be supplied to each of a plurality of cylinders between the plurality of cylinders is equal to or greater than an allowable value” is occurring.
2. Description of Related Art
A widely adopted electronic fuel injection type internal combustion engine includes at least one fuel injection valve in each cylinder or in each intake port communicating with the corresponding cylinder. Thus, when the characteristics of the fuel injection valve of a particular cylinder become “the characteristics such that a smaller amount of fuel than the instructed fuel injection amount is injected”, only the air-fuel ratio of the mixture to be supplied to the particular cylinder becomes significantly leaner. As a result, the degree of imbalance in air-fuel ratio between the cylinders increases. In other words, a state, in which an imbalance in cylinder-specific air-fuel ratio that is the air-fuel ratio of the mixture to be supplied to each cylinder (the imbalance in air-fuel ratio between cylinders) is occurring, is brought about. The imbalance in air-fuel ratio between cylinders occurs also when, for example, the characteristics of the fuel injection valve of a particular cylinder become “the characteristics such that a larger amount of fuel than the instructed fuel injection amount is injected”.
In this specification, the cylinder, for which the air-fuel ratio of the mixture to be supplied has a normal value (in general, near the stoichiometric air-fuel ratio) is referred to as “the non-imbalance cylinder” or “the normal cylinder”. The air-fuel ratio of the mixture to be supplied to the non-imbalance cylinder is referred to also as “the air-fuel ratio of the non-imbalance cylinder” or “the air-fuel ratio of the normal cylinder”. On the other hand, the cylinder, to which the mixture whose air-fuel ratio deviates from the air-fuel ratio of the mixture to be supplied to the non-imbalance cylinder is supplied, is referred to also as “the imbalance cylinder”. The air-fuel ratio of the mixture to be supplied to the imbalance cylinder is referred to also as “the air-fuel ratio of the imbalance cylinder”.
When the imbalance in air-fuel ratio between the cylinders occurs, the exhaust gas from the imbalance cylinder and the exhaust gas from the non-imbalance cylinder are alternately discharged into the exhaust passage. Thus, the air-fuel ratio of the exhaust gas varies relatively significantly, so that the output value of the air-fuel ratio sensor that is disposed in the exhaust gas joining portion, at which the exhaust gases from a plurality of cylinders join, also varies significantly. Thus, a conventional abnormality determination system acquires the trace length of the output value of the air-fuel ratio sensor, compares the trace length with the “reference value that varies according to the engine speed”, and, based on the result of comparison, determines whether the imbalance in air-fuel ratio between cylinders is occurring (see U.S. Pat. No. 7,152,594, for example).
The parameter, such as the trace length of the output value of the air-fuel ratio sensor, that is determined based on the output value of the air-fuel ratio sensor and reflects “the magnitude of the variation of the air-fuel ratio of the exhaust gas that increases as the degree of imbalance in cylinder-specific air-fuel ratio increases” is referred to also as the air-fuel ratio imbalance index value or the imbalance determination parameter.
The imbalance in air-fuel ratio between cylinders occurs not only when the degree of imbalance, between cylinders, in the amount of injected fuel (cylinder-specific fuel injection amount) to be supplied to each cylinder increases (when the imbalance in fuel injection amount between cylinders occurs) because of the abnormal characteristics of the fuel injection valve of a particular cylinder but also when the degree of imbalance, between cylinders, in the amount of air to be supplied to each cylinder (cylinder-specific intake air amount) increases (when the imbalance in intake air amount between cylinders occurs). Also when the imbalance in intake air amount between cylinders occurs, the air-fuel ratio of the exhaust gas varies relatively significantly and the air-fuel ratio imbalance index value therefore increases.
Thus, when the air-fuel ratio imbalance index value increases, it is impossible to determine whether the cause is that “the imbalance in fuel injection amount between cylinders is occurring” or “the imbalance in intake air amount between cylinders is occurring” because an abnormality occurs in the parts of the air intake system, etc. This will be described in detail with reference to FIGS. 1A and 1B.
(A1), (A2), and (A3) of FIG. 1A respectively show “the intake air amount, the fuel injection amount, and the air-fuel ratio” of each cylinder when “the amount of air taken into a particular cylinder (fourth cylinder in FIG. 1A)” becomes twice “the amount of air taken into each of the remaining cylinders (first to third cylinders)”.
(B1), (B2), and (B3) of FIG. 1B respectively show “the intake air amount, the fuel injection amount, and the air-fuel ratio” of each cylinder when “the amount of fuel injected through the fuel injection valve of the particular cylinder (fourth cylinder in FIG. 1B)” becomes half of “the amount of fuel injected through the fuel injection valve of each of the remaining cylinders (first to third cylinders)”.
Note that (A1) and (B1) of FIGS. 1A and 1B show the intake air amount of each of the other cylinders when the intake air amount of the first cylinder is assumed to be “1”. (A2) and (B2) of FIGS. 1A and 1B show the fuel injection amount of each of the other cylinders when the fuel injection amount of the first cylinder is assumed to be “1”. In addition, (A3) and (B3) of FIGS. 1A and 1B show the air-fuel ratio of each of the other cylinders when the air-fuel ratio of the first cylinder is assumed to be “1”.
As shown in (A1), (A2), and (A3) of FIG. 1A, when “the imbalance in intake air amount between cylinders” such that the intake air amount of the fourth cylinder becomes twice the intake air amount of each of the other cylinders occurs, the air-fuel ratio of the fourth cylinder becomes twice the air-fuel ratio of each of the other cylinders. As shown in (B1), (B2), and (B3) of FIG. 1B, when “the imbalance in fuel injection amount” such that the fuel injection amount of the fourth cylinder becomes half of the fuel injection amount of each of the other cylinders occurs, the air-fuel ratio of the fourth cylinder becomes twice the air-fuel ratio of each of the other cylinders. As a result, in either of these cases, the air-fuel ratio imbalance index value is substantially the same.
In conventional abnormality determination systems, however, “the inter-cylinder air intake amount variation abnormality” and “the inter-cylinder fuel injection amount variation abnormality” are not distinguished, and therefore, even when the inter-cylinder air intake amount variation abnormality is occurring actually, it is determined that “the inter-cylinder fuel injection amount variation abnormality” is occurring, for example.